COACHING FRATERNITY: Retired manager Tony La Russa has become close with the Harbaughs — he joined father Jack (above) at an Indiana hoops game last week — and advised Jim on the 49ers’ QB situation. Photo: AP

When Jim Harbaugh faced the most crucial decision of his coaching career — whether to stick with young, impressive quarterback Colin Kaepernick or give the 49ers’ starting job back to the recovered Alex Smith — he talked it over with someone who possessed zero pro football experience but more than enough credibility on this sort of dilemma.

“He mentioned what he was thinking about,” legendary baseball manager Tony La Russa told The Post in a telephone interview. “He thought [going with Kaepernick] gave him his best chance to win going forward.

“That’s what they pay him for. I told him, ‘You’re the decision-maker. They pay you for your best judgment. You trust your gut. You don’t cover your butt.’ ”

La Russa, who retired in 2011 after managing 33 seasons with the White Sox, A’s and Cardinals, emphasized Harbaugh “wasn’t running it by me.” He doesn’t want any undue credit for the move that arguably put the 49ers in Super Bowl XLVII Sunday against the Ravens and their coach John Harbaugh, Jim’s older brother.

Yet the fact that Harbaugh kept La Russa in the loop on such an important, sensitive matter shows the coaching fraternity routinely transcends individual sports. Even if they speak different languages, there can be a universal code.

“You share with coaches, no doubt about it,” said La Russa, who has the third-most wins by a manager (2,728) in baseball history. “I’m so pleased for the family and each of the two coaches that they’re experiencing coaching in the championship. I temper that because Jim beat [Packers head coach] Mike McCarthy and John beat a very great friend in Bill [Belichick, the Patriots].”

The 68-year-old La Russa stands as a particularly strong proponent of bonding with his peers across sports. He is notably tight with fellow retirees Bobby Knight and Bill Parcells and also regards Mavericks head coach Rick Carlisle as a confidant.

Fittingly, he became friendly with Harbaugh through Indiana basketball coach Tom Crean, who is married to Jim and John Harbaugh’s sister, Joani. La Russa makes his home in the San Francisco area, giving him easy access to Jim since he took over Stanford in 2007 and then graduated to the 49ers in 2011.

“I’m mostly a sounding board,” La Russa said. “I haven’t done anything there to influence anything. I just listen and answer the questions.”

La Russa also has met the Harbaugh parents, Jackie and Jack, and he has exchanged text messages with John Harbaugh. Despite knowing Jim better than his brother, La Russa insisted he won’t take sides Sunday.

“It’s hard to root against John and root against the family,” La Russa said. “I just take the same position as mom and dad.”

Jim Harbaugh and La Russa have discussed what it’s like to be in the championship game, on their sports’ center stage. La Russa took the A’s and Cardinals to three World Series apiece and won the Fall Classic three times, including his final two visits in 2006 and 2011.

“They’re all connected,” La Russa said of the different sports. “When you get to the playoffs of whatever sport … there are the same kind of potential distractions. The one thing you learn quickly is that you only can get beat by the distractions if [you let them get to you].

“What you really want to do is value the feeling of competition. You want each player, each member of the coaching staff to enjoy that challenge.”

There was no indication La Russa itches to return to the dugout; he has enjoyed his role as a special assistant to baseball commissioner Bud Selig. However, by serving as a Jedi Master of sorts for younger coaches, he still can enjoy a taste of those old sensations.

And when a move like sticking with Kaepernick pays off as sensationally as it has for Jim Harbaugh? La Russa obviously won’t get credited with a win for offering his counsel to Harbaugh. But how about a save?